Coating treatment



United States Patent O 3,406,106 COATING TREATMENT Willard S. Garwood, Dearhorn, and John B. Saxton, Detroit, Mich., assignors to Ford Motor Company, Dearborn, Mich., a corporation of Delaware No Drawing. Continuation-in-part of application Ser. No. 336,037, Jan. 6, 1964. This application June 27, 1967, Ser. No. 649,094

4 Claims. (Cl. 204-37) ABSTRACT OF THE DISCLOSURE Heat treating anodized aluminum coatings in an atmosphere having a dew point of at least 150 F. prevents cracking and crazing of the anodized coating, especially coatings that have been sealed prior to heat treatment by immersion in dichromate solutions, boiling water, or other sealing materials. The dew point must be maintained above about 150 F. until the article is cooled below about 160 F. Heat treatment at temperatures exceeding 300 F. have been accomplished without cracking or crazing the anodized coating.

SUMMARY OF THE INVENTION This application is a continuation-in-part of U.S. Ser. No. 336,037, filed Ian. 6, 1964, now abandoned.

Oxide coatings are produced on aluminum and aluminum alloys for functional purposes such as increased corrosion resistance, electrical insulation, abrasion hardness, chemical inertness and adsorption, or for purely decorative purposes since the anodized surface generally is visually aesthetic. One of the main advantages of the anodized coating results from the adsorptive properties of the coating. Various materials can be adsorbed by the coating for specialized functional or decorative effects. In addition, ordinary paints can be applied to the anodized surface.

Most anodized coatings on aluminum or aluminum alloys are sealed by immersion in boiling water or potassium dichromate solutions or exposure to superheated steam after the oxide formation but before further processing. A typical sealing process is disclosed in British Patent 595,365, Dec. 3, 1947. These sealing treatments decrease coating porosity, thereby increasing corrosion resistance and also apparently increasing the strength of the coating. Subsequent heat treatment of articles having a sealed, anodized coating heretofore cracked or crazed the coating, thereby destroying its corrosion resistance, electrical insulation, chemical inertness and aesthetic appearance. Such heat treatment is desirable, for example, where finishes such as thermally curing paints requiring relatively high temperatures are applied to the anodized coating itself or an unanodized surface of an article having an anodized coating.

This invention provides a process for heat treating aluminum or aluminum alloy articles having a sealed anodized coating that prevents cracking or crazing of the anodized coating during the heat treatment. In any process that involves subjecting an article having a sealed anodized aluminum coating to a temperature exceeding about 150 F., this invention is carried out by maintaining sufficient water vapor in the atmosphere contacting the anodized coating to produce a dew point of at least 150 F. in that atmosphere until the anodized surface has been cooled to a temperature below about 160 F.

Cracking or crazing tendencies of anodized surfaces depend on numerous factors. For example, thicker coatings tend to crack or craze more readily than thinner coatings. The age of a coating also is a factor. Maintaining the dew point at 150-160 F. for anodized coatings Patented Oct. 15, 1968 about .30 mil thick or less during heat treatment at temperatures up to about 250 F. generally prevents all serious cracking or crazing. At higher heat treatment temperatures of 275-285 F. coatings up to about .40 mil thick exhibited no cracking or crazing when the dew point was maintained at about 212 F. In each case, the dew point must be maintained relatively high until the anodized surface is cooled to less than about 160 F.

DETAILED DESCRIPTION Example I A panel of 5252 aluminum having an anodized oxide coating of .280 mil was sealed by dipping in a two percent potassuim dichromate solution and then baked under normal conditions at 200 F. for thirty minutes. Extensive crazing resulted from the baking and the crazing completely destroyed the corrosion resistance and appearance of the anodized coating. Similar panels exposed to 190 F. in a trichloroethylene degreaser exhibited equivalent crazing.

The temperature of the furnace then was increased to 220 F. and the dew point of the atmosphere therein was increased to 212 F. Several panels of 5252 aluminum having equivalent coating thicknesses were baked in this oven for thirty minutes. The panels were cooled in the oven While being exposed to the humid atmosphere to about F. before removal. No cracking or crazing resulted.

Example II A 5457 aluminum panel having an anodized coating .388 mil thick was sealed and then baked at 285 F. for thirty minutes. The panel was cooled while exposed to the humid atmosphere to below 150 F. and then removed. No cracking or crazing occurred. A similar panel having a coating thickness of .366 mil baked at 275 F. in a dry atmosphere exhibited extensive crazing.

Example HI A 5252 aluminum panel was sealed and painted with a black enamel. Its anodized coating was .29 mil thick. The panel was baked at 250 F. for fifty minutes in an atmosphere having a dew point of 150 F. After baking, the panel was cooled to 140 F. before being removed from the humid atmosphere. No cracking or crazing was observed.

Example IV A 5252 aluminum panel sealed and painted as in Example III and having an anodized coating of .36 mil was baked at 250 F. for fifty minutes while exposed to an atmosphere having a dew point of F. The panel was cooled to 140 F. before its removal from the humid atmosphere. A very slight crazing was noted when the panel was removed, but this crazing was not apparent after an hour. A panel prepared in exactly the same Way and having exactly the same coating thickness showed very extensive crazing when baked at 250 F. for fifty minutes without the humid atmosphere.

The mechanism by which the humid atmosphere prevents cracking or crazing is not known but it is believed the high humidity increases the ductility of the anodized coating so stresses caused by differential thermal expansion are relieved by plastic flow instead of cracking. Sealing treatments apparently add moisture to the coatings but this moisture probably is driven off during the initial stages of heat treatment in the absence of a humid atmosphere and thereby renders the coating susceptible to cracking or crazing.

Numerous finishes can be cured on articles having anodized aluminum coatings using the process of this invention without incurring cracking or crazing. Typical finishes include coatings having film forming components of cellulose lacquers such as cellulose nitrate, vinyl lacquers such as the vinyl chlorides and acrylics, drying oils such as linseed or other natural oils, alkyd varnishes and enamels, polymerizable materials such as phenolic varnishes and enamels, and epoxys, etc. In addition to articles made of pure aluminum, the process also is effective when aluminum alloy articles having anodized coatings are subjected to a heat treatment. Generally, unsealed anodized coatings are sufficiently porous to resist cracking or crazing during heat treatment, so the process of this invention is most effective on sealed coatings. Where unsealed coatings are subjected to extremely high temperatures of over 400 F., however, exposing the anodized coatings to a humid atmosphere assists in inhibiting any crazing tendency.

Thus this invention provides a process that permits heat treating an article having an anodized aluminum coating at temperatures exceeding 300 F. without cracking or crazing the anodized coating. In addition to its usefulness during general manufacturing of articles having anodized coatings, the process is extremely useful when baking repainted automobiles and trucks having anodized aluminum decorative trim.

What we claim is:

1. In a process that involves subjecting an article having a sealed anodized aluminum coating to a temperature exceeding about 150 F., the improvement comprising maintaining suflicient water vapor in the atmosphere contacting the anodized coating to produce a dew point of at least 150 F. in said atmosphere until said anodized coating has been cooled to a temperature below about 160 F., thereby inhibiting any crazing tendency of the anodized coating.

2. The process of claim 1 in which the article is maintained at a temperature between 150 F. and 300 F. for a time sufficient to cure a finish thereon.

3. The process of claim 2 in which the anodized coating was sealed by treatment with steam, boiling water, or dichromate solutions.

4. The process of claim 1 in which the article is mounted on a vehicle passing through a paint baking oven.

References Cited UNITED STATES PATENTS 2,231,086 2/1941 Muller et a1. 204-58 2,294,334 8/ 1942 Filbert 1486 2,880,148 3/1959 Evangelides 204-35 3,210,184 10/1965 Uhlig 961 FOREIGN PATENTS 595,365 12/1947 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner.

W. VAN SISE, Assistant Examiner. 

